Severe pulmonary hypertension, including primary pulmonary hypertension (PPH), is an important clinical problem with few clinical treatment options. The chronic, intravenous infusion of prostacyclin (PGI2) has been established as the treatment of choice for patients with PPH. It is now clear that long-term benefits occur which obviate the need for transplant in many cases. The physiological effects of prostacyclin on platelet behavior, vascular tone control, and cell proliferation are well established; however, we do not know whether prostacyclin effects the vascular remodeling in chronic pulmonary hypertension. Our overall hypothesis is that prostacyclin, through membrane-receptor dependent and independent mechanisms, is an important modulator of pulmonary vascular remodeling. We have demonstrated loss of the prostacyclin receptor (PGIR) protein in the smooth muscle cells of precapillary resistance arteries in patients with PPH. We postulate that impairment of the prostacyclin signal transduction contributes to pulmonary vascular remodeling. We have generated transgenic animals with selective pulmonary prostacyclin synthase (PGIS) overexpression. These animals are protected from the development of hypoxic pulmonary hypertension, and show no acute vasoconstriction or chronic vascular remodeling. In contrast, PGIR knockout (KO) mice, in response to hypoxia, develop rapid pulmonary hypertension accompanied by vascular remodeling. Microarray analysis of the lungs from the transgenic animals demonstrates a change in the global pattern of gene expression, which may be responsible for the "protected" phenotype, including changes in PPARs and COX-2. Our underlying concept is that PGI2 exhibits both membrane-receptor mediated and nuclear-receptor-mediated actions. These alternative mechanisms could include direct effects on gene expression, signaling pathways not yet recognized, or changes in the level of other eicosanoids. Our goal is to examine, using both animal models and cell systems, the effects of PGIS and PGIR on vascular smooth muscle cell (VSMO) growth and differentiation. In Specific Aim 1, we will determine whether pulmonary vascular tone and remodeling are mediated through the PGI2 receptor using bitransgenic mice with PGIS overexpression, but lacking PGIR. Specific Aim 2 is designed to define the effect of PGIS and PGIR on the growth and remodeling of vascular smooth muscle cells. The results of this work are designed to elucidate new potential therapeutic targets for treating pulmonary hypertension, and broaden our understanding of vascular pathology in general.